Lenslet arrays have been used to concentrate light, imaged on a photodetector plane by a photographic objective, into smaller areas to allow more of the incident light to fall on the photosensitive area of the photodetector array and less on the insensitive areas between the pixels. This has been described in papers such as "Visible Array Detectors" by Timothy J. Tredwell, from Handbook of Optics, Vol. 1, Fundamentals Techniques, & Design, Second Edition, Chapter 22, pp. 32-34. These lenslet arrays are centered directly above the corresponding photosensor and are not designed to look at different portions of the field of view independently. Rather, they concentrate the light from an existing image, formed by the photographic objective, into the pixel aperture.
In U.S. Pat. No. 4,994,664, entitled, "Optically Coupled Focal Plane Arrays Using Lenslets And Multiplexers" by Veldkamp, an array of diffractive lenslets is used to concentrate incident light onto an array of photosensors in order to allow for location of amplifying circuitry in areas between photosensor sites. These lenslets are centered over the photosensitive sites and are formed on the opposite side of the photosensor array on a silicon substrate, the use of the silicon substrate prevents them from imaging visible light onto the photosensors since silicon does not transmit in the visible wavelengths. That invention is not be able to work over the visible wavelength range since the element is composed of all diffractive optical power and suffers from severe chromatic aberrations.
In U.S. Pat. No. 5,233,174, entitled, "Wavefront Sensor Having A Lenslet Array As A Null Corrector" by Zmek, teaches an array of diffractive lenslets with decenters that are adjusted to eliminate the local monochromatic wavefront tilt from a specific optic under test in an interferometric or Hartman type test. A Hartman test is used to certify the surface quality of various optics. If the optics under test falls within the acceptance criteria, the wavefront incident on the sensor array will form centered light spots on predetermined pixels. If the wavefront is other than the desired wavefront, the light spots will be incident on different pixel elements. That invention is not applicable to the current invention since the centered lenslets are not looking at angularly displaced sections of a larger field of view. It is also not applicable to white light applications due to the chromatic aberrations of the diffractive lenslets.
U.S. Pat. No. 5,340,978, entitled, "Image-Sensing Display With LCD Display Panel And Photosensitive Element Array" Rostoker et al., briefly describes an array of decentered lenses which form an image of a segment of the field of view. These lenses are widely separated and do not include a method for limiting a field of view seen by a group of pixels. The use of the widely separated pixels will greatly increase costs of the sensor since there will be fewer sensor arrays fabricated on a given size substrate or wafer which will cause a decreased yield of finished sensor arrays for a given manufacturing process. There is no discussion of the trade-off between the focal length of the lens array and the angular subtense of the pixel's field of view. If the focal length of the lenslets is too short, light from one angular location which is incident on one group of pixels will also be within the field of view of an adjacent group. For very short focal lengths the corresponding pixel dimension required for an equivalent angular resolution will be so small as to not be able to be fabricated with lithographic processes. If the pixel dimension is reduced too much the light gathering area of the pixel will be so small as to not generate a reliably measurable number of electrons for a given incident intensity. Rostoker does not envision the use of diffractive/refractive hybrids for achromatization. That patent uses an array of uniformly shaped lenslets, while in the present invention the utility of varying the surface profile of the lenslets as a function of their radial position in the lenslet array allows for a higher level of aberration correction at any given location. In the current invention the lenses are abutted to each other and an opaque baffle is placed over the photodetector to limit the field of view of each pixel. The referenced patent uses one lenslet per group of three color pixels. In the current invention it is shown to be advantageous to form a small array of pixels for each lenslet if the focal length of each lenslet is adjusted appropriately (increased).
The invention disclosed in U.S. Pat. No. 5,471,515, to Fossum, et. al., entitled "Active Pixel Sensor with Intra-Pixel Charge Transfer," converts the photogenerated charge stored under the photogate of a semiconductor photosensor into a voltage by transferring the charge to a sense node (typically a capacitor) located within the active pixel unit cell. Fossum then utilizes dual sample correlated double sampling of the voltage based signal to reduce signal noise and eliminate the effect of dark current from the photosensor. The voltage associated with the image exposure is then subtracted from the voltage associated with a read during a dark sample by a voltage differencing amplifier located at the end of the row or column of the photosensors. By using appropriate row and column select data lines, a subsection of the array can be read out without the need to read out the entire image array. The Fossum invention does not however enable an increase in the overall sensitivity of the photosensor (CCD detector) elements, nor does it envision the utilization of an array optic type structure to form an image of different segments of a field of view, although the patent does disclose the use of a lens array for concentrating light on the active pixel. Fossum is performing most of the signal processing in a voltage amplification mode, whereas the present invention utilizes the advantages of a current mode signal processing. In addition, the present invention provides for the digitization and storage of the digital image data at each photosensor site.
In U.S. Pat. No. 5,004,901, entitled "Current Mirror Amplifier for use in an Optical Data Medium Driving Apparatus and Servo Circuit" by Yoshimoto, et. al., a photogenerated current from an optical disk tracking and read sensor is amplified in fixed steps by a switchable series of current mirrors where the current mirrors achieve current multiplication through the use of output stages that incorporate either multiple output transistors with the bases of the output transistors connected in parallel, or by the use of output transistors with emitter areas that are integral multiples of the emitter areas of the input side transistor. The purpose of Yoshimoto's invention is to allow the utilization of received photocurrents with a large dynamic range by multiplying the input current by an adjustable ratio where the multiple current ratios are selected through a switchable network of differential amplifiers. Yoshimoto's invention is not related to the field of array image sensors and requires the use of a switchable array of differencing amplifiers. Yoshimoto's invention does not integrate the current from the photosensor and the current is continuously generated by received light from the laser light emitted by the optical disk head. The sensor is not exposed to an image as in the current invention, but is used in a continuous optical disk position monitoring mode. Yoshimoto does not utilize dual slope correlated double sampling for noise reduction as disclosed in the present invention. Yoshimoto does not make any mention of the use of array optics with a field of view which varies as a function of radial position in the sensor array.